(1) Field
The invention relates to a hotplate with lifting elements for the lifting of a workpiece to be heated with the hotplate relative to the hotplate.
(2) Description of Related Art
Hotplates of this type are more preferably employed in laminating presses for manufacturing photovoltaic modules.
During laminating a thin usually film-like layer combines with a carrier material. In many cases, for example during the manufacture of photovoltaic modules, it is necessary to carry out the laminating process at elevated temperatures (hot laminating). Here, the workpiece to be processed (i.e. the carrier material covered with the layer to be joined) is generally heated on the support surface of a hot plate to a predetermined temperature and subsequently pressed.
For the laminating process it is important that the temperature distribution on the support surface of the hotplate is as even as possible. Furthermore, it must be possible to temporarily interrupt the heat supply which can for example be achieved by lifting the workpiece relative to the support surface. With some applications the laminating process is conducted in a vacuum. With applications of this type it is practical to arrange and operate the hotplate in an evacuatable chamber.
From EP 0 678 357 A1 a laminating press with a hotplate arranged in an evacuatable chamber is known for example. The hotplate comprises a multiplicity of lifting elements each of which is arranged in a vertical guide hole in a heatable plate and can be extended from the respective guide hole beyond a support surface of the plate in order to be able to lift a workpiece with respect to the support surface. The respective lifting elements can be extended by means of pneumatically actuatable pistons which are arranged in cylinders which extend in the interior of the plate below the support surface. For actuating the respective piston the pressure differential between the evacuatable chamber and the surroundings is utilised, wherein all lifting elements which are covered by the respective workpiece are automatically lifted when a vacuum is generated in the chamber. This hotplate has a number of disadvantages. Because of the many cylinders which have to be created in the interior of the plate for the pistons the manufacture of the hotplate is expensive. Furthermore, the multiplicity of cylinders and the airflows created when actuating the cylinders result in an uneven temperature distribution in the region of the support surface. In addition, the lifting of the workpiece is connected to the establishment of a vacuum in the chamber. The time which is available for heating a workpiece can thus not be chosen independently of the establishment of the vacuum and thus not freely. A further disadvantage must be seen in that the pneumatic actuation of the pistons renders simultaneous lifting of the lifting elements more difficult. In order to enforce simultaneous lifting of the lifting elements the lifting elements and pistons can be connected with one another. This would make the manufacture of the hotplate even more expensive.
From EP 1 550 548 A1 a further laminating press with a hotplate arranged in an evacuatable chamber is known. The hotplate is arranged at a distance from the base of the chamber and comprises a multiplicity of vertical bores. In order to be able to lift a workpiece resting on the hotplate a multiplicity of lifting devices is provided each of which consists of a pin and a drive for the respective pin. The drives are individually arranged on the base of the chamber below the hotplate in such a manner that the respective pins can be guided or extended through the bores in the hotplate. For example pneumatic cylinders or mechanical drives are provided as drives. This arrangement of hotplate and lifting devices is also associated with several disadvantages. On the one hand because of the multiplicity of the drives, expensive measures are required in order to be able to operate and control the drives in the vacuum, in particular measures for sealing the drives and the chamber. Furthermore, the assembly of the lifting devices and the hotplate are expensive, in particular in the case of relatively large plates and a large number of lifting devices. In addition, the maintenance of the drives is made more difficult, more so since these are not easily accessible below the plate. Furthermore, simultaneous extending of all pins is associated with considerable expenditure because of the multiplicity of the drives, more so since all drives have to be synchronised.